1. Field of the Invention
The present invention relates to a magneto-optical head apparatus. More particularly, the present invention relates to a magneto-optical head apparatus for use in a magneto-optical recording and/or reproducing apparatus which is suitable for achieving reduction in size.
2. Background of the Invention
An optical head apparatus is used to record or reproduce an information signal onto or from an optical recording medium, e.g., a magneto-optical recording medium. A typical optical head apparatus is arranged as shown in FIG. 1. It should be noted that in the following description the optical head apparatus will be explained by way of one example in which a disc-shaped optical recording medium (hereinafter referred to simply as "optical disc") is used as an optical recording medium.
Referring to FIGS. 1 and 2, the optical head apparatus 1 has a semiconductor laser 2 as a light source, a reflecting mirror 3, an objective lens 5, a beam splitter 6, a Wollaston prism 7, and a photodetector 8. The optical head apparatus 1 is, although not shown, provided with an actuator for driving the objective lens 5 in both focusing and tracking directions. The semiconductor laser 2 outputs a light beam on the basis of a drive signal from a driver circuit (not shown) such that, when an information signal is to be recorded onto an optical disc 4, the semiconductor laser 2 outputs a light beam of high output level, whereas, when an information signal is to be read out from the optical disc 4, the semiconductor laser 2 radiates a light beam of low output level. The reflecting mirror 3 may be a totally reflecting mirror or other reflecting mirror. The reflecting mirror 3 is disposed on the optical axis of the objective lens 5 at a tilt angle of 45.degree. with respect to the Z-axis direction in FIG. 1, that is, the optical axis of the light beam emitted from the semiconductor laser 2. The objective lens 5 converges the light beam reflected by the reflecting mirror 3 on the recording surface of the optical disc 4. The objective lens 5 is an aspherical lens made of a material transparent to a light beam. The beam splitter 6 separates the light beam emitted from the semiconductor laser 2 and a light beam through the objective lens 5 from each other, and deflects the optical path of the light beam passing through the objective lens 5 at 90.degree.. The beam splitter 6 is, for example, a polarization beam splitter. The Wollaston prism 7 generates a plurality of light beams from the light beam separated by the beam splitter 6. The Wollaston prism 7 is formed by bonding together two prisms made of an optical material having optical anisotropy, as disclosed, for example, in U.S. Pat. No. 4,771,414. The Wollaston prism 7 is attached to one output surface of the beam splitter 6, that is, one side surface of the beam splitter 6, as shown in FIG. 2. The photodetector 8 has a plurality of light receiving portions for receiving the light beams, respectively, which are generated by the Wollaston prism 7. The photodetector 8 is disposed at a position where the light beams outputting from the Wollaston prism 7 are focused.
It should be noted that the optical disc 4 is rotationally driven at a constant linear velocity or a constant angular velocity by a spindle motor 9.
In the optical head apparatus 1 arranged as described above, the light beam emitted from the semiconductor laser 2 passes through the beam splitter 6 and impinges on the reflecting mirror 3 where the optical path of the light beam is deflected at 90.degree. so that the light beam is led to the objective lens 5. The objective lens 5 converges the light beam emitted from the semiconductor laser 2 on the recording surface of the optical disc 4. The light beam reflected by the recording surface of the optical disc 4 is led into the optical head apparatus 1 through the objective lens 5. The light beam irradiating the optical head apparatus 1 through the objective lens 5 is separated from the light beam emitted from the semiconductor laser 2 by the beam splitter 6, while the optical path thereof is deflected at 90.degree. by the beam splitter 6, and thus the light beam outputs from the Wollaston prism 7 in the form of a plurality of light beams, which are received by the respective light receiving portions of the photodetector 8. On the basis of an output signal from the photodetector 8, a reproduced signal of the information signal recorded on the optical disc 4 is generated, together with various error signals based on a focusing error, a tracking error, etc. A servo signal for driving the above-described actuator is generated according to the focusing error signal and the tracking error signal.
In the optical head apparatus 1, as shown in FIG. 2, the optical path, or the optical axis, which extends from the semiconductor laser 2 to the reflecting mirror 3 via the beam splitter 6, and the optical path, or the optical axis of the light beam, which extends from the reflecting mirror 3 to the photodetector 8 via the beam splitter 6 are parallel to the recording surface of the optical disc 4. In other words, the semiconductor laser 2, the beam splitter 6, the Wollaston prism 7 and the photodetector 8 are disposed in a plane parallel to the recording surface of the optical disc 4. In contrast, the optical path, or the optical axis, which extends from the reflecting mirror 3 to the objective lens 5 is perpendicular to the recording surface of the optical disc 4.
The optical head apparatus 1 is fed from the inner periphery toward the outer periphery or vice versa of the optical disc 4, that is, in the X-axis direction in FIG. 2, by a feed mechanism (not shown). During the feed of the optical head apparatus 1, the spot of light beam converged on the recording surface of the optical disc 4 by the objective lens 5 moves on one straight line passing through the center of tracks on the recording surface of the optical disc 4, which is driven to rotate by the spindle motor 9, as shown in FIG. 2.
In the optical head apparatus 1 arranged as described above, since the semiconductor laser 2, the beam splitter 6, the Wollaston prism 7 and the photodetector 8 are disposed in a plane parallel to the recording surface of the optical disc 4, when the optical head apparatus 1 is fed toward the inner periphery of the optical disc 4, particularly when it is fed to the innermost peripheral region, the optical head apparatus 1 and the spindle motor 9 are likely to interfere with each other. That is, since the photodetector 8 is disposed at a position in the optical head apparatus 1 that is closer to the spindle motor 9, as shown in FIG. 2, when the optical head apparatus 1 is fed to the innermost peripheral region of the optical disc 4, a portion of the optical head apparatus 1 where the photodetector 8 is provided is likely to collide with the spindle motor 9.
One approach to solve the above problem is to dispose the photodetector 8 at the opposite side as viewed in FIG. 2, that is, at a position symmetric with the illustrated position with respect to the optical axis of the light beam emitted from the semiconductor laser 2. In this case, however, when the optical head apparatus 1 is fed to the outermost peripheral region of the optical disc 4, it projects outward from the outer peripheral edge of the optical disc 4, resulting in an increase in the overall size of a recording and/or reproducing apparatus that uses the optical head apparatus 1.
Further, in the optical head apparatus 1 as shown in FIGS. 1 and 2, the light beam reflected by the optical disc 4 is separated and deflected by the beam splitter 6 before being applied to the photodetector 8 through the Wollaston prism 7. Accordingly, the optical path length from the beam splitter 6 to the photodetector 8 becomes relatively long, resulting in an increase in the overall size of the optical head apparatus 1. Further, since the number of constituent elements of the optical head apparatus 1 is large, a large number of steps are required for assembly and adjustment, which leads to a rise in cost.